Transmit gating in a wireless communication system

ABSTRACT

A method and system for communicating a frame of information according to both a continuous transmit format and a discontinuous transmit format. The present invention contemplates transmitting one or more data rates in either a continuous transmit mode ( 814 ) or a discontinuous transmit mode ( 812 ). The present invention contemplates continuous transmission only for certain data rates, and selection between continuous and discontinuous transmission for other data rates ( 810 ). Frames transmitted in the discontinuous transmit mode may be transmitted at a higher transmit power than in the continuous transmit mode ( 820 ). In one embodiment, the information is transmitted at a fifty-percent duty cycle during the second half of the frame when in the discontinuous transmit mode ( 808 ). During periods of non-transmission, an alternative system may be searched for as a possible candidate for hard handoff ( 816 ).

CROSS-REFERENCE TO PROVISIONAL APPLICATION

This application claims the benefit of U.S. provisional application No.60/075,211, filed on Feb. 19, 1998. The disclosure of this provisionalapplication is incorporated herein by reference. This application claimbenefit to Provisional Application No. 60/092,537 Jul. 13, 1988.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communications. More particularly, thepresent invention relates to a novel and improved method and apparatusfor transmitting variable rate data in a wireless communication system,and for assisting a hard handoff.

II. Description of the Related Art

The use of code division multiple access (CDMA) modulation techniques isone of several techniques for facilitating communications in which alarge number of system users are present. Other multiple accesscommunication system techniques, such as time division multiple access(TDMA) and frequency division multiple access (FDMA) are known in theart. However, the spread spectrum modulation techniques of CDMA havesignificant advantages over these modulation techniques for multipleaccess communication systems. The use of CDMA techniques in a multipleaccess communication system is disclosed in U.S. Pat. No. 4,901,307,entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USINGSATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of thepresent invention, and incorporated by reference herein. The use of CDMAtechniques in a multiple access communication system is furtherdisclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FORGENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”,assigned to the assignee of the present invention and incorporated byreference herein.

CDMA by its inherent nature of being a wideband signal offers a form offrequency diversity by spreading the signal energy over a widebandwidth. Therefore, frequency selective fading affects only a smallpart of the CDMA signal bandwidth. Space or path diversity is obtainedby providing multiple signal paths through simultaneous links from amobile user through two or more cell-sites. Furthermore, path diversitymay be obtained by exploiting the multipath environment through spreadspectrum processing by allowing a signal arriving with differentpropagation delays to be received and processed separately. Examples ofpath diversity are illustrated in U.S. Pat. No. 5,101,501 entitled“METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN ACDMA CELLULAR TELEPHONE SYSTEM”, and U.S. Pat. No. 5,109,390 entitled“DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assignedto the assignee of the present invention and incorporated by referenceherein.

A method for transmission of speech in digital communication systemsthat offers particular advantages in increasing capacity whilemaintaining high quality of perceived speech is by the use of variablerate speech encoding. The method and apparatus of a particularly usefulvariable rate speech encoder is described in detail in U.S. Pat. No.5,414,796, entitled “VARIABLE RATE VOCODER”, assigned to the assignee ofthe present invention and incorporated by reference herein.

The use of a variable rate speech encoder provides for data frames ofmaximum speech data capacity when the speech encoder is providing speechdata at a maximum rate. When the variable rate speech encoder isproviding speech data at a less that maximum rate, there is excesscapacity in the transmission frames. A method for transmittingadditional data in transmission frames of a fixed predetermined size,wherein the source of the data for the data frames is providing the dataat a variable rate, is described in detail in U.S. Pat. No. 5,504,773,entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FORTRANSMISSION”, assigned to the assignee of the present invention andincorporated by reference herein. In the above mentioned patentapplication, a method and apparatus is disclosed for combining data ofdiffering types from different sources in a data frame for transmission.

In frames containing less data than a predetermined capacity, powerconsumption may be lessened by transmission gating a transmissionamplifier such that only parts of the frame containing data aretransmitted. Furthermore, message collisions in a communication systemmay be reduced if the data is placed into frames in accordance with apredetermined pseudorandom process. A method and apparatus for gatingthe transmission and for positioning the data in the frames is disclosedin U.S. Pat. No. 5,659,569, entitled “DATA BURST RANDOMIZER”, assignedto the assignee of the present invention and incorporated by referenceherein.

A useful method of power control of a mobile in a communication systemis to monitor the power of the received signal from the wirelesscommunication device at a base station. In response to the monitoredpower level, the base station transmits power control bits to thewireless communication device at regular intervals. A method andapparatus for controlling transmission power in this fashion isdisclosed in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FORCONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONESYSTEM”, assigned to the assignee of the present invention andincorporated by reference herein.

In a communication system that provides data using a QPSK modulationformat, very useful information can be obtained by taking the crossproduct of the I and Q components of the QPSK signal. By knowing therelative phases of the two components, one can determine roughly thevelocity of the wireless communication device in relation to the basestation. A description of a circuit for determining the cross product ofthe I and Q components in a QPSK modulation communication system isdisclosed in U.S. Pat. No. 5,506,865, entitled “PILOT CARRIER DOTPRODUCT CIRCUIT”, assigned to the assignee of the present invention andincorporated by reference herein.

There has been an increasing demand for wireless communications systemsto be able to transmit digital information at high rates. One method forsending high rate digital data from a wireless communication device to acentral base station is to allow the wireless communication device tosend the data using spread spectrum techniques of CDMA. One method thatis proposed is to allow the wireless communication device to transmitits information using a small set of orthogonal channels. Such a methodis described in detail in co-pending U.S. Pat. No. 6,396,804, entitled“HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM”, assigned to theassignee of the present invention and incorporated by reference herein.

In the just-mentioned application, a system is disclosed in which apilot signal is transmitted on the reverse link (the link from thewireless communication device to the base station) to enable coherentdemodulation of the reverse link signal at the base station. Using thepilot signal data, coherent processing can be performed at the basestation by determining and removing the phase offset of the reverse linksignal. Also, the pilot data can be used to optimally weigh multipathsignals received with different time delays before being combined in arake receiver. Once the phase offset is removed, and the multipathsignals properly weighted, the multipath signals can be combined todecrease the power at which the reverse link signal must be received forproper processing. This decrease in the required receive power allowsgreater transmission rates to be processed successfully, or conversely,the interference between a set of reverse link signals to be decreased.

While some additional transmit power is necessary for the transmissionof the pilot signal, in the context of higher transmission rates theratio of pilot signal power to the total reverse link signal power issubstantially lower than that associated with lower data rate digitalvoice data transmission cellular systems. Thus, within a high data rateCDMA system, the E_(b)/N₀ gains achieved by the use of a coherentreverse link outweigh the additional power necessary to transmit pilotdata from each wireless communication device.

An additional benefit of the reverse link described in this co-pendingapplication is that it generates less amplitude modulation (AM)interference due to its continuous-transmit nature. Thus, users withsensitive electronic equipment such as hearing aids and pacemakers willexperience less interference than with a discontinuous transmit reverselink. Another example of the use of continuous transmission to reduce AMinterference is given in co-pending U.S. Pat. No. 6,205,190, filed Apr.29, 1996, entitled “SYSTEM AND METHOD FOR REDUCING INTERFERENCEGENERATED BY A CDMA COMMUNICATIONS DEVICE”, assigned to the assignee ofthe present invention and incorporated herein by reference.

However, when the data rate is relatively low, acontinuously-transmitted pilot signal on the reverse link contains moreenergy relative to the data signal. At these low rates, the benefits ofcoherent demodulation and reduced interference provided by acontinuously-transmitted reverse link pilot signal may be outweighed bythe decrease in talk time and system capacity in some applications. Amethod and system is needed to provide flexibility in reverse linktransmission format as needed to optimize these tradeoffs.

Further, a communications device may need to go into hard handoff from afirst system to a second system. If discontinuous transmission ispossible, the device may search for the second system during the periodsof non-transmission, while maintaining contact with the first systemduring periods of transmission.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and system forcommunicating a frame of information according to both a continuoustransmit format and a discontinuous transmit format. In one aspect ofthe present invention, a method is disclosed for transmitting frames ofinformation. The method includes transmitting information continuouslythroughout the frame when in a continuous transmit mode and the frame isof a first data rate of a plurality of data rates; and transmitting theinformation discontinuously in the frame when in a discontinuoustransmit mode and the frame is of the first data rate. Thus, the presentinvention contemplates transmitting one or more data rates in either acontinuous transmit mode or a discontinuous transmit mode.

The method may further include transmitting the information continuouslythroughout the frame when the frame is of a second data rate of theplurality of data rates. Thus, the present invention contemplatescontinuous transmission only for certain data rates, and selectionbetween continuous and discontinuous transmission for other data rates.

In one embodiment of the present invention, the first data ratecorresponds to a first transmit power and the second data ratecorresponds to a second transmit power, and the first transmit power isless than the second transmit power. In this embodiment, the methodincludes transmitting the frame of the first data rate at the secondtransmit power when in the discontinuous transmit mode. Thus, framestransmitted in the discontinuous transmit mode may be transmitted at ahigher transmit power than in the continuous transmit mode.

In one embodiment of the present invention, the information istransmitted at a fifty-percent duty cycle during the frame when in thediscontinuous transmit mode. This may include transmitting theinformation during a second half of the frame.

Another embodiment of the present invention includes selecting betweenthe continuous transmit mode and the discontinuous transmit mode inresponse to a transmit power of the wireless communication device. Inother words, the present invention may include selecting thediscontinuous transmit mode when the transmit power is less than apredetermined threshold. In an alternate embodiment, the presentinvention includes selecting between the continuous transmit mode andthe discontinuous transmit mode according to a user-defined preference.

The present invention also contemplates a wireless communication devicefor transmitting frames of information. The wireless communicationdevice includes a variable rate data source for generating the frames ofinformation, each of the frames of information having one of a pluralityof data rates. It also includes a transmitter for transmitting theinformation continuously throughout the frame when in a continuoustransmit mode and when the frame is of a first data rate of theplurality of data rates, and for transmitting the informationdiscontinuously in the frame when in a discontinuous transmit mode andwhen the frame is of the first data rate. Thus, the wirelesscommunication device may transmit frames of a given data rate eithercontinuously or discontinuously. A control processor selects between thecontinuous transmit mode and the discontinuous transmit mode. Thewireless communication device may implement the method of the presentinvention as summarized briefly above.

The present invention also includes a method for receiving a frame ofinformation in a wireless receiver, wherein the information may becontinuously present throughout the frame or discontinuously present inthe frame. This method includes filtering the frame of information in asliding window filter to produce a sliding window phase estimate signal,filtering the frame of information in a block window filter to produce ablock window phase estimate signal, and selecting between the slidingwindow phase estimate signal and the block window phase estimate signalin response to whether the information is continuously present in theframe.

In one embodiment of the present invention, the method includesselecting the sliding window phase estimate signal when the informationis continuously present in the frame, and selecting the block windowphase estimate signal when the information is discontinuously present inthe frame. Additionally, the method may include selecting the blockwindow phase estimate signal before and after a phase discontinuity inthe frame.

The present invention further contemplates a wireless receiver forreceiving a frame of information wherein the information may becontinuously present throughout the frame or discontinuously present inthe frame. The wireless receiver includes a sliding window phaseestimator for filtering the frame of information in a sliding window toproduce a sliding window phase estimate signal, a block window phaseestimator for filtering the frame of information in a block window toproduce a block window phase estimate signal, and a multiplexer forselecting between the sliding window phase estimate signal and the blockwindow phase estimate signal in response to whether the information iscontinuously present in the frame. The wireless receiver may implementthe method briefly described above.

Additionally, the present invention discloses a method, in a wirelesscommunication system, for communicating a frame of information between awireless communication device and a wireless base station in acontinuous transmit mode and a discontinuous transmit mode. The methodincludes transmitting, from the wireless communication device, theinformation continuously throughout the frame when in the continuoustransmit mode, and transmitting, from the wireless communication device,a first message notifying the wireless base station of an intention totransmit in a discontinuous mode. In response, the base stationtransmits a second message acknowledging the intention to transmit inthe discontinuous mode, and the wireless communication device transmitsthe information discontinuously in the frame when in the discontinuoustransmit mode, and in response to the second message.

In one embodiment, the method further includes demodulating the frame ofinformation according to a continuous transmit format when theinformation is continuously present throughout the frame, anddemodulating, the frame of information according to a discontinuoustransmit format when the information is discontinuously present in theframe.

The present invention further contemplates a wireless communicationsystem for communicating a frame of information in a continuous transmitmode and a discontinuous transmit mode. The wireless communicationsystem includes a wireless communication device and a wireless basestation that implement the method described briefly above.

In a final aspect of the present invention, a method and apparatus aredisclosed for facilitating hard handoff from a first system to a secondsystem. The device searches for the second system during the periods ofnon-transmission, while maintaining contact with the first system duringperiods of transmission.

Gating is supported for rate sets 3, 4, 5 and 6. When a frame is gated,only the symbols within the second half of the frame are sent. Thismeans that symbols 6144 through 12287, numbering from 0, aretransmitted. During gating, the maximum frame rate is rate 1/2.

Normally, the blocks are transmitted using continuous transmission, withthe exception of the rate 1/8 frame which is gated. The continuoustransmission reduces the interference in the audio band. The rate 1/8frame is gated because it improves the reverse link capacity and themobile station talk time relative to when continuous transmission isused.

However, rate set 3, 4, 5 and 6 may be commanded into a mode where onlyrate 1/8, rate 1/4, and rate 1/2 frames are transmitted and aretransmitted using gated transmission. This mode is used to allow themobile station time to retune its receiver and search for systems usingfrequencies and other technologies (e.g. AMPS and GSM).

During gating, the second half of the frame is transmitted for thefollowing reasons. First, the gating needs to be either in the firsthalf or the second half of the frame. If it were not, then the framewould not contain a contiguous 10 milliseconds for searching. Second,the transmitted portion of the frame needs to occur later in the framein order to allow the mobile station time to estimate the differencebetween the measured and expected forward signal to noise ratio.Therefore, during gating, the second half of the frame is sent.

In addition, rate set 3, 4, 5 and 6 may be commanded into a mode whereall frames are transmitted using continuous transmission. This mode isused by mobile stations that may need to further reduce audio bandinterference.

A mobile station commanded into gated mode for searching will becommanded to periodically gate N frames out of M frames on the forwardlink and reverse link simultaneously, starting at system time T. Thevalues of N and M depend on the technology being searched and the numberof channels being searched.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a functional block diagram of an exemplary embodiment of thetransmission system of the present invention embodied in wirelesscommunication device 50;

FIG. 2 is a functional block diagram of an exemplary embodiment ofmodulator 26 of FIG. 1;

FIG. 3 illustrates four graphs of the average energy transmitted bytransmitter 28 of FIG. 1 over a single frame for four different datarates;

FIG. 4 is a functional block diagram of selected portions of a basestation 400 in accordance with the present invention;

FIG. 5 is an expanded functional block diagram of an exemplary singledemodulation chain of demodulator 404 of FIG. 4; and

FIG. 6 is an expanded functional block diagram of an exemplary pilotfilter that uses a sliding window estimator in combination with a blockwindow estimator.

FIG. 7 is a block diagram of apparatus for assisting in hard handoff.

FIG. 8 is a block diagram of a method for assisting in hard handoff.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a functional block diagram of an exemplary embodimentof the transmission system of the present invention embodied in wirelesscommunication device 50. It will be understood by one skilled in the artthat the methods described herein could be applied to transmission froma central base station (not shown) as well. It will also be understoodthat various of the functional blocks shown in FIG. 1 may not be presentin other embodiments of the present invention. The functional blockdiagram of FIG. 1 corresponds to an embodiment that is useful foroperation according to the TIA/EIA Standard IS-95C, also referred tocommercially as cdma2000. Other embodiments of the present invention areuseful for other standards including Wideband CDMA standards anddual-mode CDMA/GSM standards. These other embodiments differ somewhat inthe formatting of data for transmission, but still include the inventiveprinciples described herein.

In the exemplary embodiment of FIG. 1, the wireless communication devicetransmits a plurality of distinct channels of information which aredistinguished from one another by short orthogonal spreading sequencesas described in the aforementioned U.S. Pat. No. 6,396,804. Fiveseparate code channels are transmitted by the wireless communicationdevice: 1) a first supplemental data channel 38, 2) a time multiplexedchannel of pilot and power control symbols 40, 3) a dedicated controlchannel 42, 4) a second supplemental data channel 44 and 5) afundamental channel 46. The first supplemental data channel 38 andsecond supplemental data channel 44 carry digital data which exceeds thecapacity of the fundamental channel 46 such as facsimile, multimediaapplications, video, electronic mail messages or other forms of digitaldata. The multiplexed channel of pilot and power control symbols 40carries pilots symbols to allow for coherent demodulation of the datachannels by the central base station and power control bits to controlthe energy of transmissions to wireless communication device 50. Controlchannel 42 carries control information to the central base station suchas modes of operation of wireless communication device 50, capabilitiesof wireless communication device 50 and other necessary signalinginformation. Fundamental channel 46 is the primary channel used to carrythe primary information signal from the wireless communication device tothe central base station. In the case of speech transmissions, thefundamental channel 46 carries the speech data.

Supplemental data channels 38 and 44 are encoded and processed fortransmission by means not shown and provided to modulator 26. Powercontrol bits are provided to repetition generator 22 which providesrepetition of the power control bits before providing the bits tomultiplexer (MUX) 24. In multiplexer 24 the redundant power control bitsare time multiplexed with pilot symbols and provided on line 40 tomodulator 26.

Message generator 12 generates necessary control information messagesand provides the control message to CRC and tail bit generator 14. CRCand tail bit generator 14 appends a set of cyclic redundancy check bitswhich are parity bits used to check the accuracy of the decoding at thecentral base station and appends a predetermined set of tail bits to thecontrol message. The message is then provided to encoder 16 whichprovide forward error correction coding upon the control message. Theencoded symbols are provided to interleaver 18 which reorders thesymbols in accordance with a predetermined interleaver format. Theinterleaved symbols are provided to repetition generator 20 whichrepeats the reordered symbols to provide additional time diversity inthe transmission. The interleaved symbols are provided on line 42 tomodulator 26.

Variable rate data source 1 generates variable rate data. In theexemplary embodiment, variable rate data source 1 is a variable ratespeech encoder such as described in aforementioned U.S. Pat. No.5,414,796. Variable rate speech encoders are popular in wirelesscommunications because their use increases the battery life of wirelesscommunication devices and increases system capacity. TheTelecommunications Industry Association has codified the most popularvariable rate speech encoders in such standards as Interim StandardIS-96 and Interim Standard IS-733. These variable rate speech encodersencode the speech signal at four possible rates referred to as fullrate, half rate, quarter rate or eighth rate according to the level ofvoice activity. The rate indicates the number of bits used to encode aframe of speech and varies on a frame by frame basis. Full rate uses apredetermined maximum number of bits to encode the frame, half rate useshalf the predetermined maximum number of bits to encode the frame,quarter rate uses one quarter the predetermined maximum number of bitsto encode the frame and eighth rate uses one eighth the predeterminedmaximum number of bits to encode the frame.

Variable rate date source 1 provides the encoded speech frame to CRC andtail bit generator 2. CRC and tail bit generator 2 appends a set ofcyclic redundancy check bits which are parity bits used to check theaccuracy of the decoding at the central base station and appends apredetermined set of tail bits to the control message. The frame is thenprovided to encoder 4 which provide forward error correction coding onthe speech frame. The encoded symbols are provided to interleaver 6which reorders the symbols in accordance with a predeterminedinterleaver format. The interleaved symbols are provided to repetitiongenerator 8 which provided repetition of the reordered symbols toprovide additional time diversity in the transmission. The interleavedsymbols are provided on line 46 to modulator 26.

In the exemplary embodiment, modulator 26 modulates the data channels inaccordance with a code division multiple access modulation format andprovides the modulated information to transmitter (TMTR) 28 whichamplifies and filters the signal and provides the signal throughduplexer 30 for transmission through antenna 32.

FIG. 2 illustrates a functional block diagram of an exemplary embodimentof modulator 26 of FIG. 1. The first supplemental data channel data isprovided on line 38 to spreading element 52 which covers thesupplemental channel data in accordance with a predetermined spreadingsequence. In the exemplary embodiment, spreading element 52 spreads thesupplemental channel data with a short Walsh sequence (+−−+). The spreaddata is provided to relative gain element 54 which adjusts the gain ofthe spread supplemental channel data relative to the energy of the pilotand power control symbols. The gain adjusted supplemental channel datais provided to a first summing input of summer 56. The pilot and powercontrol multiplexed symbols are provided on line 40 to a second summinginput of summing element 56.

Control channel data is provided on line 42 to spreading element 58which covers the supplemental channel data in accordance with apredetermined spreading sequence. In the exemplary embodiment, spreadingelement 58 spreads the supplemental channel data with a short Walshsequence (++++−−−−). The spread data is provided to relative gainelement 60 which adjusts the gain of the spread control channel datarelative to the energy of the pilot and power control symbols. The gainadjusted control data is provided to a third summing input of summer 56.

Summing element 56 sums the gain adjusted control data symbols, the gainadjusted supplemental channel symbols and the time multiplexed pilot andpower control symbols and provides the sum to a first input ofmultiplier 72 and a first input of multiplier 78.

The second supplemental channel is provided on line 44 to spreadingelement 62 which covers the supplemental channel data in accordance witha predetermined spreading sequence. In the exemplary embodiment,spreading element 62 spreads the supplemental channel data with a shortWalsh sequence (+−+−). The spread data is provided to relative gainelement 64 which adjusts the gain of the spread supplemental channeldata. The gain adjusted supplemental channel data is provided to a firstsumming input of summer 66.

The fundamental channel data is provided on line 46 to spreading element68 which covers the fundamental channel data in accordance with apredetermined spreading sequence. In the exemplary embodiment, spreadingelement 68 spreads the supplemental channel data with a short Walshsequence (++−−). The spread data is provided to relative gain element 70which adjusts the gain of the spread fundamental channel data. The gainadjusted fundamental channel data is provided to a second summing inputof summer 66.

Summing element 66 sums the gain adjusted second supplemental channeldata symbols and the fundamental channel data symbols and provides thesum to a first input of multiplier 74 and a first input of multiplier76.

In the exemplary embodiment, a pseudonoise spreading using two differentshort PN sequences (PN_(I) and PN_(Q)) is used to spread the data. Inthe exemplary embodiment the short PN sequences, PN_(I) and PN_(Q), aremultiplied by a long PN code to provide additional privacy. Thegeneration of pseudonoise sequences is well known in the art and isdescribed in detail in aforementioned U.S. Pat. No. 5,103,459. A long PNsequence is provided to a first input of multipliers 80 and 82. Theshort PN sequence PN_(I) is provided to a second input of multiplier 80and the short PN sequence PN_(Q) is provided to a second input ofmultiplier 82.

The resulting PN sequence from multiplier 80 is provided to respectivesecond inputs of multipliers 72 and 74. The resulting PN sequence frommultiplier 82 is provided to respective second inputs of multipliers 76and 78. The product sequence from multiplier 72 is provided to thesumming input of subtractor 84. The product sequence from multiplier 74is provided to a first summing input of summer 86. The product sequencefrom multiplier 76 is provided to the subtracting input of subtractor84. The product sequence from multiplier 78 is provided to a secondsumming input of summer 86.

The difference sequence from subtractor 84 is provided to basebandfilter 88. Baseband filter 88 performs necessary filtering on thedifference sequence and provides the filtered sequence to gain element92. Gain element 92 adjusts the gain of the signal and provides the gainto upconverter 96. Upconverter 96 upconverts the gain adjusted signal inaccordance with a QPSK modulation format and provides the unconvertedsignal to a first input of summer 100.

The summing sequence from summer 86 is provided to baseband filter 90.Baseband filter 90 performs necessary filtering on difference sequenceand provides the filtered sequence to gain element 94. Gain element 94adjusts the gain of the signal and provides the gain to upconverter 98.Upconverter 98 upconverts the gain adjusted signal in accordance with aQPSK modulation format and provides the upconverted signal to a secondinput of summer 100. Summer 100 sums the two QPSK modulated signals andprovides the result to transmitter 28.

FIG. 3 illustrates four graphs of the average energy transmitted bytransmitter 28 over a single frame for full rate 300, half rate 302,quarter rate 304, and eighth rate transmissions 306 and 308,respectively. As can be seen, for the full rate transmission 300, theaverage energy is equal to some predetermined maximum level, E. For halfrate transmission 302, the average energy is equal to half thepredetermined maximum level, or E/2. Likewise for quarter-ratetransmission 304, the average energy is equal to one-quarter of thepredetermined maximum level, or E/4.

For the eighth-rate transmissions 306 and 308, there are two possibletransmit energies. The first transmission 306 uses continuoustransmission at one-eighth of the predetermined maximum level, or E/8.The second transmission 308 (shown in dashed lines), uses a 50% dutycycle transmission at one-quarter of the predetermined maximum level, orE/4. In other words, the present invention provides two separatetransmission schemes for the eighth-rate frames: a continuoustransmission 306 at E/8, and a discontinuous transmission 308 at E/4. Itshould be noted that the discontinuous transmission 308 shown in FIG. 3is merely exemplary. Other duty cycles and energy values are alsocontemplated by the present invention. For example, a 25% duty cycle atand energy of E/2 may be used in one embodiment. Another embodiment usesa 50% duty cycle with the transmission occurring in the first half ofthe frame, rather than the second half of the frame as shown in FIG. 3.In yet another embodiment, the transmission start time is randomizedduring the frame. However, even in the embodiments that do not randomizethe transmission, the frame offset staggering in increments of 1.25 msthat is inherent to cdma2000 will distribute the aggregate interferencewell over a frame duration.

The amount of energy, timing, and duty cycle chosen are not limiting ofthe present invention. However, in the embodiment shown in FIG. 3, thetransmission occurs during the second half of the frame so that powercontrol can be the most accurate at the end of the frame in case thefollowing frame is at a higher data rate. And hence more critical tocontrol accurately since the higher data rate frames are transmitted athigher power and contain more information. Also in the embodiment ofFIG. 1, the interleavers 6 and 18 and repetition generators 8 and 20format the data such that transmitting only the second half of the frameensures that each of the original information bits are transmitted atleast once.

Control processor 36 controls the selection of whether the eighth-ratetransmission is continuous or discontinuous. Variable rate data source 1generates a rate indication to control processor 36, informing thecontrol processor 36 what the present data rate is. In response, controlprocessor 36 determines whether to gate transmitter 28 on and off toimplement the discontinuous transmission of the eighth-rate frames. Inone embodiment of the present invention, control processor 36 instructsmessage generator 12 to generate a message for transmission to the basestation over the control channel indicating that the wirelesscommunication device 50 intends to operate in the discontinuous mode. Inanother embodiment, this message may be a request to operate indiscontinuous mode, provided that the base station receiver can supportdiscontinuous mode transmissions.

In one embodiment of the present invention, the control processor 36 maybe programmed to always transmit eighth-rate frames according to thediscontinuous mode shown as dashed line 308 of FIG. 3. In anotherembodiment, the control processor 36 may dynamically determine whetherto transmit continuously or discontinuously according to the presenttransmit power of transmitter 28. Since the AM interference caused bydiscontinuous transmission is proportional to the amplitude of thetransmitted signal, the control processor 36 may compare the presenttransmit power to a predetermined threshold. If the transmit power isgreater than the predetermined threshold, the control processor 36 doesnot gate the transmitter 28, resulting in continuous transmission. Ifthe transmit power is less than or equal to the predetermined threshold,the control processor 36 does gate the transmitter 28, resulting indiscontinuous transmission. In such an embodiment, the present transmitpower may be determined by any means known in the art. For example, bymeasuring the output power of transmitter 28 with a conventional signallevel detector circuit (not shown), or by accumulating power controlcommands from the base station, or by monitoring automatic gain controlsignals being sent to the transmitter 28. Each of these powermeasurement techniques is well known in the art and will not be expandedupon herein.

In another embodiment of the present invention, the control processor 36determines whether to transmit continuously or discontinuously accordingto user-defined preferences. For example, a menu option may be presentedto a user on a graphical display (not shown), allowing the user toenable or disable discontinuous transmission. This embodiment would beparticularly useful to persons using sensitive electronic equipment suchas hearing aids and pacemakers to allow them to program their wirelesscommunication device to always perform continuous transmission. Thisallows the user to make their own decision about the tradeoff betweenbattery life and potentially dangerous AM interference. Still anotherembodiment allows discontinuous transmission during voice calls, anddisables discontinuous transmission during data calls.

Typical wireless communication device power amplifiers use significantamounts of current. Also, other transmit signal processing componentsconsume power. An example of the current consumption for variouscomponents in the transmitter 28 is shown in TABLE I below.

TABLE I Function Current (mA) Power Amplifier Bias Current 110-130 mAPower Amplifier Driver Current 42 mA DAC, filtering, upconverter, AGC 40mA Total 202 mA

As can be seen from TABLE I above, approximately 202 mA of current maybe switched out during discontinuous transmission in a typical wirelesscommunication device. A typical variable rate data source 1, duringnormal 30 human speech, will produce eighth-rate frames about 63% of thetime. So the potential average current savings for the example of TABLEI is about 63% eighth-rate frames*50% duty cycle*202 mA=64 mA. This is asignificant amount of current savings in a typical wirelesscommunication device where the total current consumption isapproximately 320 mA at 100% duty cycle. In this example, discontinuoustransmission of eighth-rate frames at a 50% duty cycle yields about a25% increase in talk time.

In addition to the increase in talk time, a system capacity benefit isalso realized by the present invention. As is known in the art, thestrength of the reverse link pilot signal is driven primarily by theneed to track the carrier phase and timing of the reverse link waveform.For most of the time during voice calls a typical wireless communicationdevice is transmitting eighth-rate frames, and therefore is transmittingmostly pilot energy. By turning both the pilot and data signals offduring low rate frames, the present invention enhances system capacity.

For example, if we assume that the required traffic component E_(b)/N₀is 1.6 dB per antenna at 9600 bps, 0.1 dB per antenna at 1500 bps, andthe required pilot component E_(c)/N₀ is −22.1 dB per antenna, we findthe pilot power fraction shown below in TABLE II.

TABLE II Traffic Traffic Average Pilot Pilot Data Rate E_(b)/N₀ (dB)E_(c)/N₀ (dB) Power (bps) per antenna per antenna (%) 9600 1.6 −22 36%Continuous 1500 0.1 −22 86% 50% duty cycle 0.1 −25 76% 1500

Using the approximations shown above in TABLE II, gating the 1500 bpsframes at the 50% duty cycle reduces the average voice call E_(c)/N₀ by0.85 dB for 8 kbps vocoder operation.

By operating at the exemplary 50% duty cycle for eighth-rate frames, theability to maintain power control on the reverse link and forward linkis affected. The update rate is reduced by a factor of two. For example,the update rate in a cdma2000 system may be reduced from 800 times persecond to 400 times per second. This tends to cause an increase in theframe error rate for the eighth-rate frames. However, the increase incapacity and talk time gained by the present invention may outweigh thisdecrease in power control accuracy in many applications. Additionally,in one embodiment of the present invention, the transmit period (i.e.,the time that the transmitter 28 is gated “on”) is arranged to occur atthe end of the frame so that power control is most accurate at the frameboundary where the data rate may suddenly increase for the next frame.

Turning now to FIG. 4, a functional block diagram of selected portionsof a base station 400 in accordance with the present invention. Reverselink RF signals from the wireless communication device 50 (FIG. 1) arereceived by receiver (RCVR) 402, which downconverts the received reverselink RF signals to an baseband frequency. The baseband signal is thendemodulated by demodulator 404. Demodulator 404 is further describedwith reference to FIG. 5 below.

In the exemplary embodiment of FIG. 4, demodulator 404 has multipleoutputs 405A-405N, each corresponding to a different one of the logicalchannels modulated by modulator 26 of FIG. 1. For example, output 405Acorresponds to the control channel 42 of FIG. 1, and output 405Ncorresponds to the fundamental channel 46 of FIG. 1. Demodulator 404typically will have other demodulated signal outputs. However, forclarity and simplicity, only the control channel 405A and fundamentalchannel 405N are shown in FIG. 4.

The control channel 405A data is de-interleaved by deinterleaver 406,decoded by decoder 408 and CRC checked by CRC checker 410. Each of thesefunctional blocks 406-410 performs a complementary function as theircounterparts in blocks 14-18 of FIG. 1. The control channel data is thenpassed to control processor 412 for further processing. For example, thecontrol channel data may include a message from the wirelesscommunication device 50 indicating that it either intends, or isrequesting, to operate in discontinuous mode. In response to thismessage, control processor 412 directs message generator 424 (whichincludes forward link data formatting) to generate a reply message tothe wireless communication device 50, acknowledging reception of theintention or request message. The acknowledgment message is thenmodulated by modulator 422 and transmitted by transmitter (TMTR) 420.

The fundamental channel 405N is de-interleaved by deinterleaver 414,decoded by decoder 416 and CRC checked by CRC checker 418. Each of thesefunctional blocks 414-418 performs a complementary function as theircounterpart blocks 2-6 of FIG. 1. The fundamental channel data is thenpassed to other subsystems (not shown) in the base station 400 forfurther processing as required.

When control processor 412 receives a request message from wirelesscommunication device 50 to operate in discontinuous mode, it configuresdeinterleavers 406, 414, decoders 408, 416, and CRC checkers 410, 418for operation in discontinuous mode. In one embodiment, this means thatdeinterleavers 406, 414, decoders 408, 416, and CRC checkers 410, 418ignore the portions of the frame that do not contain data.

Turning now to FIG. 5, an expanded functional block diagram of anexemplary single demodulation chain of demodulator 404 is shown. In thepreferred embodiment, demodulator 404 has one demodulation chain foreach information channel. The exemplary demodulator 404 of FIG. 5performs complex demodulation on signals modulated by the exemplarymodulator 26 of FIG. 1. As previously described, receiver (RCVR) 402downconverts the received reverse link RF signals to a basebandfrequency, producing I and Q baseband signals. Despreaders 502 and 504respectively despread the I and Q baseband signals using the long codefrom FIG. 1. Baseband filters (BBF) 506 and 508 respectively filter theI and Q baseband signals.

Despreaders 510 and 512 respectively despread the I and Q signals usingthe PN_(I) sequence of FIG. 2. Similarly, despreaders 514 and 516respectively despread the Q and I signals using the PN_(Q) sequence ofFIG. 2. The outputs of despreaders 510 and 512 are combined in combiner518. The output of despreader 516 is subtracted from the output ofdespreader 512 in combiner 520.

The respective outputs of combiners 518 and 520 are then Walsh-uncoveredin Walsh-uncoverers 522 and 524 with the Walsh code that was used tocover the particular channel of interest in FIG. 2. The respectiveoutputs of the Walsh-uncoverers 522 and 524 are then summed over oneWalsh symbol by accumulators 530 and 532.

The respective outputs of combiners 518 and 520 are also summed over oneWalsh symbol by accumulators 526 and 528. The respective outputs ofaccumulators 526 and 528 are then applied to pilot filters 534 and 536.Pilot filters 534 and 536 generate an estimation of the channelconditions by determining the estimated gain and phase of the pilotsignal data 40 (see FIG. 1). The output of pilot filter 534 is thencomplex multiplied by the respective outputs of accumulators 530 and 532in complex multipliers 538 and 540. Similarly, the output of pilotfilter 536 is complex multiplied by the respective outputs ofaccumulators 530 and 532 in complex multipliers 542 and 544. The outputof complex multiplier 542 is then summed with the output of complexmultiplier 538 in combiner 546. The output of complex multiplier 544 issubtracted from the output of complex multiplier 540 in combiner 548.Finally, the outputs of combiners 546 and 548 are combined in combiner550 to produce the demodulated signal of interest 405.

Of particular interest to the present invention are pilot filters 534and 536. In order to obtain a more accurate estimate of the pilot phaseand gain during reception of discontinuous transmissions, the presentinvention preferably uses a pilot filter that accounts for the180-degree phase shift at the boundary between continuous anddiscontinuous transmission in any frame. For example, in the 50% dutycycle transmission 308 (FIG. 3), the pilot filter account for the phasechange that occurs at time T/2 in each frame of length T.

One embodiment of the present invention utilizes a “sliding” filterwindow in combination with a “blocked” filter window in order to avoidimproper pilot estimation at the discontinuity boundary. The “blocked”filter is used to estimate the pilot gain and phase immediately beforeand after any discontinuities in the frame. The “sliding” filter is usedto estimate the pilot gain and phase during the remainder of the frame.An exemplary pilot filter that uses a sliding window estimator 600 incombination with a block window estimator 612 is shown in FIG. 6.

In FIG. 6, the output of either accumulator 526 of 528 is applied toshift register 602, and is also fed forward to combiner 604. In theexemplary embodiment, shift register 602 is a twelve-stage shiftregister. The shifted output of shift register 602 is subtracted fromthe fed-forward input in combiner 604 and provided to combiner 606. Theoutput of combiner 606 is delayed in delay element 608 and fed back tobe combined with the output of combiner 604 in combiner 606. The outputof delay element 608 is also provided to truncator 610 where it istruncated to be 11 bits, and provided as one selectable input tomultiplexer 614. This input to multiplexer 614 represents the slidingwindow estimate of the pilot phase and gain.

The output of either accumulator 526 or 528 is also provided to blockwindow estimator 612 which simply accumulates the signal over apredetermined period and provides an output representing the blockwindow estimate of the pilot phase and gain as a second selectable inputto multiplexer 614.

Multiplexer 614 is controlled by a select signal from control processor412 which selects between the sliding window estimate and the blockwindow estimate inputs when operating in the discontinuous transmitmode. During a predetermined period immediately before and after anydiscontinuity, control processor 412 selects the block window estimatefrom multiplexer 614. At other times during the frame, control processor412 selects the sliding window estimate from multiplexer 614. The outputof multiplexer 614 is then applied to either complex multipliers 538 and540 or 542 and 544 as shown in FIG. 5.

A slightly different embodiment of the pilot filters 534, 536 implementsa sliding window equal taps FIR filter of 2.5 ms in length. However, dueto the phase discontinuity boundaries caused by discontinuoustransmission, the window size is reduced immediately before and aftereach phase discontinuity boundary to smooth the effect of the phasediscontinuity. The filter is updated at the modulation symbol rate whichin the exemplary embodiment is one update every two chips. This resultsin the corresponding phase estimate output also having a two chipresolution. The minimum window size is preferably 1.25 ms, and thewindow size grows symbol by symbol until it reaches the sliding windowbuffer size of 2.5 ms. Other embodiments may use combinations of thetechniques described above to account for the phase discontinuityboundaries inherent in discontinuous transmission.

FIG. 7 shows the apparatus of the final aspect of present invention. ACode Division Multiple Access (CDMA) mobile station 700 includes asymbol source 702, an interleaver 704, and a transmitter 706. The symbolsource 702 may be a conventional microphone and vocoder.

The interleaver 704 is connected to receive symbols from the symbolsource 702, and is constructed to interleave them within a frame. Thetransmitter 706 is connected to receive the frame of interleavedsymbols, and is constructed to transmit it.

The apparatus further includes a gate 708 constructed to disabletransmission during a fraction F of the frame. The interleaver 704 isconstructed to repeat each symbol at least 1/F times. The gate 708 ofFIG. 7 is shown as connected directly to the transmitter 706. It couldalternatively fractionally disable transmission by manipulating theinterleaver 704. This alternative structure is more complicated and isnot preferred.

Preferably, F=1/2, so the interleaver 704 repeats each symbol at leasttwice (and preferable more times) in the frame. Thus, even though halfof the frame is not transmitted, at least one copy (and preferably morecopies) of each symbol is transmitted in each frame.

It is better for the gate 708 to be constructed to disable thetransmitter 706 during the first half of the frame rather than thesecond half. If the transmitted portion of the frame occurs later in theframe, then the mobile station can better estimate the differencebetween the measured and expected forward signal to noise ratio.

The conventional mobile station 700 includes a frame rate indicator 710,which produces an indication as to how fast the mobile station 700 istransmitting. This indication is useful for many purposes. In thepresent invention, it is applied to a selector 712. The selector 712 isconnected to receive the frame rate indication from the frame rateindicator 710. It is also constructed to selectively enable the gate 708in response to the frame rate indication. That is, it selectivelyinstructs the gate 708 to turn off the transmitter 706 during the firsthalf of the frame (enables the gate), or instructs the gate 708 to leavethe transmitter 706 on for the entire frame (disables the gate).

If desired, the selector 712 may include an adjustment mechanism 714constructed to enable the gate for all frame rate indications. This isdesirable if the mobile station is used in an area where capacity islimited. However, this gating on and off produces interference in theaudio band. When it is important to reduce audio interference, theadjustment mechanism 714 may be constructed to disable the gate for allframe rate indications. Preferably, however, the adjustment mechanism714 is constructed to enable the gate for a first predetermined set offrame rate indications 716, and to disable the gate for a secondpredetermined set of frame rate indications 718.

The apparatus may also include a mode commander 720, constructed tocommand a mode in which transmission of frames is enabled only when oneof the first (generally slower) predetermined set of frame rateindications is applied to the mode commander 720. That is, thetransmitter 706 is disabled—for the entire frame, and not just for itsfirst half—for the second (generally faster) set of frame rates. Thus,the transmitter 706 is disabled for the first half of every frame (andalso for the second half of some of the frames). This permits a receiverretuner 722 to be connected to receive a mode command from the modecommander 720. It is constructed to retune a receiver, when so commandedby the mode commander 720, during the fraction of the frame (the firsthalf) during which transmission is disabled.

The conventional mobile station 700 includes a power indicator 724,which indicates the power at which the mobile station 700 istransmitting. The present invention uses this by connecting the selector712 to receive a power indication from the power indicator 724. Theselector 712 is then constructed to selectively enable the gate 708depending on both the frame rate indication and the power indication.

FIG. 8 shows the method of operation 800 of the final aspect of thepresent invention. The present invention may thus be viewed as a method800 for operating a Code Division Multiple Access (CDMA) mobile station.The conventional method includes providing a sequence of symbols 802,interleaving each symbol within a frame 804, and transmitting the frameof interleaved symbols 806. To this, the present invention addsdisabling transmission during a fraction F of the frame 808. Theinterleaving 804 thus must include repeating each symbol at least 1/Ftimes. As before, preferably F=1/2, and preferably the fractionaldisabling of the frame transmission takes place during the first half ofthe frame.

The fractional disabling of the frame transmission is selective inresponse to a frame rate indication 810. The selective fractionaldisabling 808 may include fractionally disabling the frame transmissionat all frame rate indications 812, or may fractionally disable the frametransmission at no frame rate indication 814.

The selective fractional disabling 808 may include fractionallydisabling the frame transmission for a first predetermined set of framerate indications, and excludes fractionally disabling the frametransmission for a second predetermined set of frame rate indications.The method may further include commanding a mode 816 in whichtransmission of frames is enabled only for the first predetermined setof frame rate indications. In this case, it also includes retuning areceiver 818, when the mode is so commanded, during the fraction of theframe during which transmission is disabled.

The selective fractional disabling may also include fractionallydisabling the frame transmission depending on both the frame rateindication 810 and a power indication 820.

Thus, the present invention provides a method and apparatus for transmitgating in a wireless communication system which allows the wirelesscommunication device to operate either in continuous or discontinuoustransmit modes.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

I claim:
 1. A method of transmitting a reverse link signal, the methodcomprising: generating a first frame of data having a first data rate ofa plurality of possible data rates; selecting a discontinuous transmitmode based on the first data rate; and transmitting the reverse linksignal in the discontinuous transmit mode, wherein the reverse linksignal comprises a pilot signal and a data signal, and wherein the datasignal comprises the frame of data.
 2. The method of claim 1 furthercomprising modulating the pilot signal and the data signal to providethe reverse link signal.
 3. The method of claim 1 further comprising:generating a second frame of data having a second data rate of theplurality of possible data rates; and selecting a continuous transmitmode based on the second data rate.
 4. The method of claim 1 wherein thefirst data rate is eighth-rate.
 5. The method of claim 1 wherein thetransmitting comprises transmitting the reverse link signal at a 50%duty cycle.
 6. The method of claim 5 wherein the transmitting comprisestransmitting the reverse link signal during a contiguous periodrepresenting half of a frame period.
 7. The method of claim 5 whereinthe transmitting comprises transmitting the reverse link signal duringan earlier half of a 20 millisecond frame period.
 8. The method of claim5 wherein the transmitting comprises transmitting the reverse linksignal during a latter half of a 20 millisecond frame period.
 9. Themethod of claim 1 wherein the reverse link signal further comprisespower control bits, and wherein the transmitting the reverse link signalin the discontinuous transmit mode results in a reduction in the updaterate of the power control bits.
 10. The method of claim 1 furthercomprising determining whether to transmit discontinuously, wherein theselecting is further based on the determining.
 11. The method of claim10 wherein the determining is based on a user-defined preference. 12.The method of claim 10 wherein the determining is based on a transmitpower of a mobile station transmitter.
 13. The method of claim 10wherein the determining is based on a discontinuous mode messagereceived from a base station.
 14. A reverse link transmission apparatuscomprising: variable rate data source configured to generate a firstframe of data having a first data rate of a plurality of possible datarates; modulator configured to modulate a pilot signal and the firstframe of data to provide a reverse link signal; transmitter configuredto transmit the reverse link signal, wherein the reverse link signalcomprises a pilot signal and a data signal, and wherein the data signalcomprises the frame of data; and control processor configured to selecta discontinuous transmit mode based on the first data rate and tocontrol gating of the transmitter based on the selecting a discontinuoustransmit mode.
 15. The apparatus of claim 14 wherein the controlprocessor is further configured to select a continuous transmit modebased on a second data rate of a frame generated by the variable ratedata source, and to control gating of the transmitter based on theselecting a continuous transmit mode.
 16. The apparatus of claim 14wherein the control processor is further configured to select adiscontinuous transmit mode when the first frame of data is aneighth-rate frame.
 17. The apparatus of claim 14 wherein the controlprocessor is configured to control gating of the transmitter in thediscontinuous transmit mode at a 50% duty cycle.
 18. The apparatus ofclaim 14 wherein the control processor is configured to control gatingof the transmitter in the discontinuous transmit mode by enablingtransmission during half of a frame period.
 19. The apparatus of claim14 wherein the control processor is configured to control gating of thetransmitter in the discontinuous transmit mode by enabling transmissionduring an earlier half of a 20 millisecond frame period.
 20. Theapparatus of claim 14 wherein the control processor is configured tocontrol gating of the transmitter in the discontinuous transmit mode byenabling transmission during a latter half of a 20 millisecond frameperiod.
 21. The apparatus of claim 14 wherein the modulator is furtherconfigured to modulate power control bits to provide the reverse linksignal, and wherein the transmitter is configured to transmit fewerpower control bits per frame in the discontinuous transmit mode than ina continuous transmit mode.
 22. The apparatus of claim 14 wherein thecontrol processor is further configured to determine whether to transmitdiscontinuously and to base the selecting on the determining.
 23. Theapparatus of claim 22 wherein the control processor is configured tobase the determining on a user-defined preference.
 24. The apparatus ofclaim 22 wherein the control processor is configured to base thedetermining on a transmit power of a mobile station transmitter.
 25. Theapparatus of claim 22 wherein the control processor is configured tobase the determining on a discontinuous mode message received from abase station.
 26. A reverse link transmission apparatus comprising:means for generating a first frame of data having a first data rate of aplurality of possible data rates; means for selecting a discontinuoustransmit mode based on the first data rate; and means for transmitting areverse link signal in the discontinuous transmit mode, wherein thereverse link signal comprises a pilot signal and a data signal, andwherein the data signal comprises the frame of data.